FORMATION OF ISOHEXYL ALKYLAROMATIC HYDROCARBONS FROM AROMATIZATION-REARRANGEMENT OF TERPENOIDS IN THE SEDIMENTARY ENVIRONMENT - A NEW CLASS OF BIOMARKER

Seven alkylaromatic hydrocarbons with an isohexyl and a methyl substit uent group on adjacent ring carbons have been identified in a suite of crude oils derived from a range of locations, source types, palaeoenv ironments, and ages. Hitherto unidentified series of isohexylalkylbenz enes and isohexylalkynaphthalenes have been observed with concentratio ns ranging from 10 to 565 mu g/g. Isohexylalkylbenzenes occur in crude oils ranging in age from Cambrian to Tertiary suggesting both a bacte rial/algal and higher plant origin, whereas isohexylalkylnaphthalenes were restricted to crude oils of Permian to Tertiary age and are exclu sively associated with specific higher plant precursors. The unique st ructure of these compounds has been shown to result from rearrangement and A-ring opening of diterpenoid and triterpenoid natural products d uring aromatization. The structural features that are necessary for th is process to occur are a terpenoid A-B ring system with a gem-dimethy l group at C-4 and an angular methyl at C-10. Support for a ring openi ng process concomitant with aromatization was provided by laboratory e xperiments involving dehydrogenation of terpenoid natural products. An alysis of these reaction products showed that isohexyl alkylaromatic c ompounds, along with fully aromatized compounds with intact carbon ske letons were formed.